Why most scholars say that tawbah is the first step in the preparation of Ramadan?

Tawbah is a cleaning up process. During Ramadan, we are trying to absorb and to take things in. So we are like vessels for taqwa, the Qur'an and the things of Ramadan. That vessel needs to be cleaned up so that it can accept the new content in a new way. Otherwise, if the vessel remains dirty, then whatever we put inside will be dirty. This is the analogy which is usually given concerning the need for tawbah before Ramadan. It is one of the primary goals of Ramadan. It makes sense that we should, in our preparation for Ramadan, be involved in purifying ourselves of what went before.

We have in the Qur'an,

"O You who believe, turn to Allah in sincere repentance, so perhaps your Lord will expiate your sins for you, and enter you into paradise, under which rivers flow…"[1]

And the Prophet (ﷺ) has said,

“One who repents from sin is like one without sin.”[2]

References:

[1] [Qur’an 66:8]

[2] [Narrated by Ibn Maajah, 4250; classed as saheeh by al-Albaani in Saheeh Ibn Maajah]